Flywheel hybrid systems (KERS)

When Max Mosley announced at the British Grand Prix in 2006 that he wanted F1 cars to develop regenerative braking systems he must have hoped he was talking to the right people to achieve his aim. However, he couldn’t have foreseen just how directly his hopes for the initiative would be turned into reality. As luck would have it, sitting in the stands listening to his comments was Jon Hilton, who now runs just the type of small, high-technology company Mosley hoped would develop the systems for Formula 1. Moreover, Hilton’s company, once led down this route by the needs of F1, is now actively pursuing applications for the system in road vehicles and being rewarded with much interest. So Mosley’s hopes are close to being realised, right through to the technology developed for F1 feeding into the cars we drive on the road. And all this has happened in less than 18 months since that revelation at Silverstone.
However, when Hilton heard those words he was a drivetrain engineer at Renault F1 in Enstone, UK and was attending the GP as a perk of the job. The team was working toward the engine freeze in F1 and they knew there would be less work for the engine team once homologation was confirmed, so Hilton though he should take a look at energy recovery. ‘Back at Renault I said, “is anybody looking at this?” and they said “no,”‘ he recalls. ‘So I asked if I could.’ He was told he could but not to spend any money.
Together with his design manager Doug Cross they started looking at every conceivable solution. ‘We even looked at a knotted rubber band from the nose of the car to the tail,’ he laughs. ‘It’s rubbish but we looked at it. It took about 15 minutes to work out it was hopeless but we looked at everything.’ Eventually they put a proposal to the team that the most promising technology was a flywheel-based system and then, should there be sufficient development budget available, an electrical system might be considered.
By then though Renault was faced with reducing its overheads and, in a round of redundancies, both Hilton and Cross were laid off, despite already being deep into the concept of energy recovery, and knowing that in 2009 F1 teams would need to have this technology fully developed and racing.
‘We sat down in the pub and said this is how big a job it is,’ Hilton recalls. ‘We worked out how much we thought it would cost to get to the point of a running demonstrator on an engine dyno – we thought we would have to get that far to sell it to anybody – and we discussed whether we had enough finance between us, and agreed we did.’ And so Flybrid Systems was born. The pair were free from Renault to work from 1 January this year and so on 2 January moved into the Silverstone Technology Centre and began work.
Opting to develop the flywheel solution they sat down and roughed out the basics of a design.

How it works
Very simply the system comprises a flywheel connected by a continuously variable transmission [CVT] to the drivetrain. If you move the CVT toward a gear ratio that would speed the flywheel up it stores energy. Conversely, if you move toward a ratio that would slow it down then it releases energy. Finally, a clutch separates the drive if the revs move beyond the limits of the system.
In itself, this is not an innovative concept and is widely used in other applications. But Flybrid’s innovations also address the need to create sufficient power storage density in a unit small enough and light enough for use in F1. To achieve this they upped the speed of the flywheel massively to 64,500rpm, which allows a smaller, lighter flywheel but also means it has to be contained in a very robust structure in case of failure. That in turn creates windage losses that sap power and produce enormous amounts of heat. The solution, Flybrid decided, was to run it in a vacuum. That cures the friction and heat issues but now raises the problem of getting power in and out without air leaking in. Electrical solutions were a possibility but the power loses in energy transfer would be too great. Instead, the team opted to develop a totally hermetic shaft seal.
Once the project was outlined it was determined that most of the challenges had off-the-shelf solutions already fully developed, apart from four areas where the company was going to have to invent its own answers. These were a flywheel construction that would remain intact at these speeds, containment that retains everything in a crash or a failure, a vacuum seal and a bearing solution.
Flybrid’s approach to these was unusual as the company set about designing three solutions to each of these problems and taking them all to manufacture in parallel. ‘We did it because it was quicker,’ says Hilton. ‘We didn’t have time to make only our favourite solution, find it didn’t work, re-draw and re-design then wait another eight weeks for the parts to be made. We didn’t feel we had two or three months to spare.’ Moreover, he feels it is not an expensive luxury. ‘Although it is expensive making hardware, it’s also expensive if it takes a long time. It consumes office rental, electricity and all the other business overheads. It’s my belief that it’s not any more expensive to do it the way we did it.’ In the event, all the solutions they tried first have worked apart from the bearing.

Solution 1 – the flywheel
The flywheel is made from carbon filament wrapped round a steel hub and weighs 5kg. Most importantly, the tensile strength of the carbon prevents it shattering under the g loads at such high speeds. Flybrid has generated a set of generic design tools and been able to consider everything from long, slim cylinders to large, thin pancakes. Further work has optimised the current design even further. Although it can store up to 60kW, the high speed means shaft torque is very low, at no more than 18Nm.

Solution 2 – the housing
Obviously, the company knows that one major injury or fatality associated with the system would spell the end of the technology and the company, so safety has been a top priority throughout the project. Its resulting containment solution is very robust and was inadvertently tested during the bearing failure mentioned earlier. In fact, damage was relatively minor, being confined to the flywheel and the containment rings, while the housing was undamaged and able to be re-used.

Solution 3 – the seal
The seal is the company’s own patentable design and Hilton is unwilling to disclose details until the patent is published late in 2008. But the achievement is something to be proud of as it can maintain a vacuum of 1×10-7bar virtually indefinitely. To get an idea of how empty that is, a molecule of air will typically have to travel 45km (30 miles) around the inside of the flywheel housing until it encounters another.

Solution 4 – the bearing
Finally, the bearing has proved the biggest challenge of all, or rather the lubrication of it has, as obviously one can’t put liquid lubricants inside a vacuum. However, the unplanned failure proved very informative as the units have always been monitored and logged during testing. ‘Now we’ve had a failure we know what they look like just before they break,’ says Hilton.

This is an important part of the reliability strategy and units can be assessed in the car with accelerometers attached to look for tell-tale signs of impending failure. A unit has also survived a simulation of the F1 nose impact test with no damage to the device.

Flybrid offers turnkey solutions for teams, complete with full servicing back up, and has already worked with one un-named F1 partner (it was believed to be Honda) who commissioned the team to design a bespoke system. Currently designs favour taking the drive from downstream of the transmission, although there are advantages to applying the drive direct to the engine. However, with the current freeze on engine design it would not be possible to re-design the crank to take the extra power.

Once the company started developing the technology, it quickly became clear to Hilton and Cross that while F1 will make some money, the real growth will be in road cars. And with this in mind they have developed flywheel designs that, whilst sacrificing a small percentage in efficiency, massively reduce their stress and become much cheaper to manufacture. The company is already in talks with car manufacturers and one potential customer is applying for government money to fund a development project with Flybrid. As Hilton points out, this is the key to working outside F1, once you have developed the technology you have the freedom to diversify into other markets.

Hope Racing (Swiss HY Tech – Hybrid)
A customer version of the motorsport system has also been developed for Le Mans Prototypes. It is a complete unit which sits between the engine and transmission as seen below on the ORECA 01 – Volkswagen setup used buy Hope Racing (engine not pictured)
The flywheel itself is situated in the silver housing at the end closest to the transmission (black section). It is not exactly the same as the F1 system described above but the principle is the same. Fitted to the Hope car is what Flybrid call the Clutched Flywheel Transmission or CFT KERS. It ran for the first time on the test bench on Friday the 25th March 2011.
At the heart of the new Flybrid KERS for Le Mans 2011, the CFT transmission is a key component of this lightweight 100 kW kinetic energy recovery system. The system uses a series of small clutches to transmit the drive between the flywheel and the main vehicle gearbox and this functionality was tested on the Flybrid full load test rig.

The CFT is said to be the first true 2nd generation KERS to come to market and represents a step change in size, weight and cost for this green technology of the future. A complete Flybrid CFT KERS for Formula One capable of 60 kW and 400 kJ per lap weighs less than 18 kg and a plan view section fits on an A4 piece of paper.
It is suitable for both racing and road car application and scales down well to small power outputs and small storage quantities such as would be required for a B class car.

The CFT transmission uses a number of discrete gears and high-speed clutches that perform a controlled slip to transmit the drive. When connected to an engine speed shaft within the vehicle transmission the three gears in the CFT KERS are multiplied by the number of gears in the main vehicle transmission to provide a large number of available overall ratios between flywheel and wheels. The efficiency of a slipping clutch depends upon the speed across it and with so many gears to choose from a high efficiency option is always available.
In a Formula One application the 21 speeds available (3 CFT KERS x 7 car gearbox speeds) mean that during a typical racing lap the efficiency will be around 64% round trip. In typical road car use the overall efficiency can be even higher so that fuel savings on the NEDC drive cycle are comparable with CVT based flywheel hybrid systems.
When in use a computer controller selects the most appropriate gear by partially engaging the high-speed clutch associated with that gear. The control system uses hydraulic pressure to close the normally open clutches and transmit the drive, seamlessly changing from one gear to another with no torque interruption as the speed across the engaged clutch reduces to near zero.
The hydraulic system is fully sealed so in F1 applications it is possible to use the normal car hydraulic system. For other applications Flybrid make and supply their own self contained hydraulic system including a very small Flybrid developed hydraulic pump and control valve block.
The CFT KERS uses conventional lubricants running at normal engine temperatures and the average heat rejection is low (just 3.0 kW average for a F1 lap) so it is easy to dissipate the heat generated by the slipping clutches. By splitting the heat to be dissipated across a number of clutches the heat management task is made much easier and the life of the wet multiplate clutches is calculated to be many thousand full power cycles.
Flybrid have filed patent applications for several features of the CFT KERS including the general arrangement as well as details of the cooling and lubrication system required for the high-speed clutches.
The CFT KERS device may be connected to the vehicle transmission in any of the locations numbered 1 through 7 shown above. When using connection locations 1 through 4 there is the advantage of multiplying the number of gears in the CFT by the number of gears in the vehicle gearbox. In locations 5 through 7 the CFT KERS may be configured with more than three gears and the round trip losses for kinetic energy recovery are lower due to the proximity of the connection to the vehicle wheels.

Sam Collins has worked for Racecar Engineering for more than a decade. His passion for racing began during his work experience in the loom shop of Williams F1 aged 16 and he has been involved in the sport ever since. Sam attended Oxford Brookes University to study Automotive Engineering and has written for many publications since, including Motorsport News and Autosport. He is Associate Editor of Racecar Engineering

FREE RACECAR ENGINEERING MONTHLY NEWSLETTER

About Us

Racecar Engineering is the world’s leading publication for motorsport technology and engineering. Every issue provides unrivalled technical analysis of everything from World Championship series including Formula 1, to grass roots racing. Using the expertise of industry professionals, we look in detail at racecar design and innovation, whilst also keeping you up to date with news and developments from all the major race series across the globe.